1. Field of the Invention
The present invention relates to an optical scanner, an optical scanning method, a scanning image forming optical system, an optical scanning lens and an image forming apparatus.
2. Discussion of the Background
An optical scanner is popularly known in relation to image forming apparatuses such as digital copying machines, optical printers, optical plotters, optical plate making machines, optical drawing machines and facsimile machines. A scanning image forming optical system, which is used in an optical scanner, condenses a luminous flux deflected by an optical deflector onto a surface to be scanned in the form of an optical spot. The optical spot condensed on the surface to be scanned by the scanning image forming optical system optically scans the surface to be scanned (practically, the photosensitive surface of a photosensitive medium comprising of a photoconductive member or the like) to write in an image.
For the scanning image forming optical system, the image surface curvature must satisfactorily be corrected so that the optical spot diameter does not vary much with the image height. To ensure satisfactory uniformity of optical scanning speed, the uniform speed characteristics such as fθ characteristic and linearity must as well be corrected satisfactorily.
For this purpose, it is often the usual practice to use a special surface shape typically represented by a non-spherical surface for the optical surface shape of an optical element (a lens or an image-forming mirror) composing the scanning image forming optical system.
Such an optical surface shape is generally formed by a forming die having a forming face (a face corresponding to the optical surface shape to be formed) formed thereon by a three-dimensional precision processing machine.
In this case, it is practically impossible to form the forming face of the die at accuracy which permits complete achievement of a designed surface shape. As a result, an actually formed optical surface shape contains a shape error to some extent relative to the designed surface shape.
In the optical surface containing such a shape error, when viewed in the main scanning direction, the shape error takes the form of a “swell” in the actual optical surface as compared with the design optical surface shape. That is, the actually formed optical surface shape becomes a shape deformed so as to have wave with reference to the design optical surface shape in the main scanning direction.
The swell as described above may be classified as follows in terms of the period of swell in the main scanning direction.
First is a low-frequency swell having a period longer than the effective width in the main scanning direction on the optical surface.
Second is a high-frequency swell having a period on a level equal to or lower than the luminous flux diameter in the main scanning direction of the incident luminous flux on the optical surface.
Third is a medium-frequency swell having a period between those of the low-frequency and the high-frequency swells in the main scanning direction of the optical surface.
A shape error of such an optical surface is a cause of occurrence of a change in the image surface position of the scanning image forming optical system.
This change in the image surface position in turn causes a change in the spot diameter of the optical spot formed on the surface to be scanned (the spot diameter referred to here is a spot diameter in the main scanning direction). When there is a change in the spot diameter in the main scanning direction according to the image height, there occurs a change in the size of dots drawn by the optical spot, which in turn causes density unevenness in which density of the written image varies in the main scanning direction. Such density unevenness is noticeably conspicuous particularly when an area is formed into a gray image area of a uniform density.
Density unevenness may be any of various frequencies ranging from low to high frequencies. When expressing the frequency of density unevenness by a spatial frequency (line/mm), the most conspicuous density unevenness to a viewer of an image, to judge from the visual properties of human eyes, in terms of spatial frequency, is believed to be density unevenness within a range of from 0.1 to 5 lines/mm.
While the low-frequency swell according to the above-mentioned classification exerts an effect on the uniform speed characteristic or change in the spot diameter, the effect on the uniform speed characteristic or density unevenness caused by the low-frequency swell is hardly noticeable by human eyes because of the long spatial period of change.
The high-frequency swell produces high-frequency density unevenness. The spatial frequency of high-frequency density unevenness, overlapping an area easily noticeable by human eyes, forms a factor causing image quality degradation.
The medium-frequency swell is a cause of degradation of the uniform-speed characteristics, and a swell of a relatively high frequency may be a cause of image quality degradation as a result of density unevenness.
Conceivable causes of a change in the image surface position bringing about a change in the spot diameter include:
(1) The image surface curvature in the main scanning direction at a design median of the scanning image forming optical system;
(2) The attachment position accuracy of optical component parts (a shift or tilting of optical elements);
(3) Processing errors of optical component parts (errors in the thickness or refractive index, excluding a shape error of the optical surface);
(4) Changes in environment (changes in the shape or in refractive index caused by a change in temperature or humidity); and
(5) Shape errors of the optical surface.
Changes in the spot diameter caused by the factors (1) to (4) above are of a relatively low frequency, and can be improved by adjustments upon attaching optical component parts, or by designing the optical systems hardly susceptible to changes caused by a change in environment. These changes hardly become cause of density unevenness of a spatial frequency area easily noticeable by human eyes.
It is therefore effective to alleviate medium-frequency and high-frequency swells from among the shape errors of the optical surface as a counter-measure against density unevenness.
The swell of the optical surface of an optical element is referred to, for example, in Japanese Patent Application Laid-open Publication No. 9-80333. The Publication proposes use of an intensity distribution changing device which reduces magnetic permeability from the optical axis toward the periphery on the light source side as a counter-measure for eliminating dark streaks (a form of density unevenness described above) occurring on a recorded image as a result of a swell of the optical surface, but it does not disclose conditions for alleviating or eliminating dark streaks through control of the swell itself.
Laser printers, laser facsimile machines and digital copying machines are strongly demanded to require a lower cost, to achieve a more compact size, and to have higher performance. Along with these requirements, cost reduction, downsizing and promotion of performance are in progress in the area of optical scanning optical systems used in these machines. In order to satisfy these requirements toward a lower cost, a smaller size and higher performance, an effective method is to reduce the number of lenses through achievement of a non-spherical optical scanning lens, and in order to achieve a non-spherical lens, it is effective to introduce a plastic lens.
It is the common practice to integrally form a plastic lens by use of a forming die, with, however, partial occurrence of swells on the formed surface. When an image is formed by optical scanning with a plastic lens having a swell, the position of the beam waist varies on the surface to be scanned, and dark streaks may be produced in the sub-scanning direction on the portion corresponding to the swell. Particularly in an optical scanner or an image forming apparatus having increased resolution and high-density gradation, dark streaks caused by the swells on the lens surface are conspicuous.
FIGS. 1 and 2 illustrate variation of the beam waist position caused by swells on the lens surface. The amount of swell amplitude is of an order of several nm to several μm, resulting in a variation of the beam waist within a range of from 0.1 to 1 mm. In FIG. 1, the swell and the variation of the beam waist position are illustrated with exaggeration. In FIG. 1, a diverging luminous flux irradiated from a laser beam source 10 is condensed by a coupling lens 12. The sectional shape of the luminous flux is rectified through an aperture 14, and the flux is further condensed only in a sub-scanning direction (y-direction perpendicular to the paper plane in FIG. 1) by a cylindrical lens 16 so that a long line image is formed in a main scanning direction (x-direction in FIG. 1) near the deflecting reflective surface of an optical deflector 20. A mirror 18 which bends the luminous flux from the cylindrical lens 16 to direct the same to the optical deflector 20 is arranged between the cylindrical lens 16 and the optical deflector 20.
The above-mentioned luminous flux entering the deflecting reflective surface of the optical deflector 20 is deflected at a uniform angular speed on the deflecting reflective surface through rotational driving of the optical deflector 20. The thus deflected luminous flux is condensed through an optical scanning lens 30, as an optical spot on the surface to be scanned 40, and scanning is performed in the x-direction at a uniform speed on the surface to be scanned 40. The scanning range of the surface to be scanned 40 is represented by W.
A swell, if present on the optical scanning lens surface, affects the beam profile, i.e., the beam intensity distribution, and this poses a problem of occurrence of dark streaks in the output image. In FIG. 1, the swell on the surface of the lens 30 is illustrated with exaggeration with reference numeral 31, and the variation of the beam waist position on the surface to be scanned 40 caused by the swell 31 is represented with exaggeration with reference numeral 41. Presence of the swell 31 on the surface of the lens 30 means production of irregularities on the surface of the lens 30. The position of the beam waist on the scanned surface 40 varies as illustrated by a curve indicated by reference numeral 41 in a portion corresponding to these irregularities. Variation of the beam waist position causes back and forth shifting of the center position of the optical spot around the surface to be scanned 40, as illustrated in FIG. 2, thus causing beam growth in diameter on the surface to be scanned 40.
Inhibition of variation of the beam waist position and growth of the beam diameter on the surface to be scanned 40 caused thereby as described above can be achieved by suppressing a peak-to-valley (PV) height of a swell of the lens surface. For this purpose, it is necessary to process the lens while strictly controlling the lens forming die in the nanometer order, and to apply high-accuracy control during forming of the lens with the die as well as during measurement of the formed lens. There is therefore a limit to the satisfaction of the required accuracy.
FIG. 3 illustrates the relationship between the spatial frequency f of a swell and the allowable amount of amplitude. As illustrated in FIG. 3, the most strict condition (1) of accuracy required for processing of a lens having a length of about several hundred mm and a height of several tens of mm is generally believed to be a high accuracy of several nanometers. This leads to a further higher accuracy required for measurement such as an ultra-high accuracy as the sub-nanometer order. In addition, since the accuracy depends also upon swell frequency, the accuracy must be controlled for each level of frequency. FIG. 3 represents a case where the accuracy is controlled at four points, (1) to (4). Satisfaction of the requirement for such an ultra-high accuracy is however subject to a certain limit in the conventional art.
In addition to the above-mentioned accuracy problem, there is a room for further study on the method of evaluation thereof. It is the conventional method for evaluating a swell of the lens surface to express the swell by a three-dimensional coordinate of x, y and z. It was however difficult to conduct an evaluation satisfying the requirement for an ultra-high accuracy.